The present invention relates to an external magnetic field correction device for automatically correcting influences of external magnetic field caused by geomagnetism on image quality, and to a CRT (cathode-ray tube) display device employing the same.
FIG. 1 is a block diagram showing a constitution of a conventional external magnetic field correction device of a CRT display device as described, for instance, in Japanese Patent Application Laid-Open No. 2-214288 (1990).
In FIG. 1, 1 denotes a CRT (cathode-ray tube), 2 a horizontal direction (the direction along the tube axial) magnetic field correction coil, 3 a vertical direction (the direction orthogonal to the tube axis) magnetic field correction coil, 4 a horizontal direction magnetic field sensing element, 5 a bias circuit, 6 an amplifying circuit, 7 an offset voltage generating circuit, 8 an adder to which an output of the amplifying circuit 6 and an output of the offset voltage generating circuit 7 are input, 9 a smoothing circuit to which an output of the adder 8 is input, and 10 a current, driving circuit for horizontal magnetic field correction to which an output of the smoothing circuit 9 is input and which output is supplied to the horizontal direction magnetic field correction coil 2.
On the other hand, 11 denotes a vertical direction magnetic field sensing element, 12 a bias circuit, 13 an amplifying circuit, 14 an offset voltage generating circuit, 15 an adder to which an output of the amplifying circuit 13 and an output of the offset voltage generating circuit 14 are input, 16 a smoothing circuit to which an output of the adder 15 is input, and 17 a current driving circuit to which an output of the smoothing circuit 16 is input and which output is supplied to the vertical direction magnetic field correction coil 3.
FIG. 2 is a block diagram showing a constitution of a conventional external magnetic field correction device of a CRT display device as described, for instance, in Japanese Patent Application Laid-Open No. 60-118879 (1985).
In FIG. 2, 4 denotes a horizontal direction magnetic field sensing element, 37 a memory (non-volatile memory) for storing a detected magnetic field intensity detected by the horizontal direction magnetic field sensing element 4, and 20 a magnetic field intensity difference detector for detecting a difference in magnetic field intensity at a predetermined position stored in the memory 37 and at a different position detected by the horizontal direction magnetic field sensing element 4.
The output, of the magnetic field intensity difference detector 20 is input, in the current driving circuit for horizontal magnetic field correction 10, and the output of the current driving circuit for horizontal magnetic field correction 10 is supplied to the horizontal direction magnetic field correction coil 2.
The Operations Will Now Be Explained.
In FIG. 1, the CRT 1 of the display device is disposed in a magnetic field that is a combination of a magnetic field generated by the horizontal direction magnetic field correction coil 2 and the vertical direction magnetic field correction coil 3 as well as an external magnetic field.
The horizontal direction magnetic field sensing element 4 is disposed into this magnetic field for converting an intensity and polarity of the magnetic field into electric signals.
The output voltage of the horizontal direction magnetic field sensing element 4 disposed in the magnetic field corresponds to a sum of horizontal components of the external magnetic field of the magnetic field in which the horizontal direction magnetic field sensing element 4 is disposed and a magnetic field generated by the horizontal direction magnetic field correction coil 2. Since the output of the horizontal direction magnetic field sensing element 4 is minute, it is amplified by the amplifier 6.
The output voltage generating circuit 7 generates a direct current which absolute value is equal to and which polarity is opposite to that of an amplified detected voltage outputted by the amplifying circuit 6 when the intensity of the magnetic field of the area in which the horizontal direction magnetic field sensing element 4 is disposed is zero.
The amplified detected voltage output by the amplifying circuit 6 when the intensity of the magnetic field in which the horizontal direction magnetic field sensing element 4 is disposed is zero varies for each individual product due to irregularities such as manufacturing irregularities of the horizontal direction magnetic field sensing element 4, irregularities in positional relationship between the horizontal direction magnetic field sensing element 4 and magnetic bodies that are employed for use, for instance, in chassis composing parts, or irregularities in parts composing the amplifying circuit. Therefore, the offset voltage generating circuit 7 needs to perform adjustment for each individual product.
The adder 8 outputs a zero value when the magnetic field intensity of the area in which the horizontal direction magnetic field sensing element 4 is disposed is zero, a negative value to a magnetic field generated when the horizontal direction magnetic field correction coil 2 is supplied with preliminarily prescribed positive current, and a positive value to a magnetic field generated when the horizontal direction magnetic field correction coil 2 is supplied with preliminarily prescribed negative current. The output of the adder 8 is smoothed by the smoothing circuit 9.
The output of the smoothing circuit 9 is inputted to the current driving circuit for horizontal magnetic field correction 10. The current driving circuit for horizontal magnetic field correction 10 is constituted by a circuit which is capable of bi-directionally supplying current, and supplies current to the horizontal direction magnetic field correction coil 2.
With this constitution, a control loop is formed in which the sum of the magnetic field generated by the horizontal direction magnetic field correction coil 2 and the horizontal directional component of the external magnetic field detected by the horizontal direction magnetic field sensing element 4 is converged to zero.
While the above explanations have been made for the horizontal direction magnetic field correction, the same applies to the vertical direction magnetic field correction.
Next, conventional the external magnetic field correction device according to FIG. 2 will be explained.
First, arranging positions of deflection coil etc. (not, shown) and electric magnetic characteristics are adjusted at manufacturing stages (process of combining/assembling with the deflection coil) of a CRT (not shown).
At this point, this adjustment as well as an external magnetic field intensity detected by a horizontal direction magnetic field sensing element. 4 are stored in a memory 37. In this condition, the CRT display device is completed and ready for shipping.
When using the CRT display device at a place to which it has been shipped, a current that is proportional to a difference between the detected value for the external magnetic field intensity at that place detected by the horizontal direction magnetic field sensing element 4 and the magnetic field intensity stored in the memory 37 is provided from the magnetic field intensity difference detector 20 to the horizontal direction magnetic field correction coil 2 through the current driving circuit for horizontal magnetic field correction 10, and by a magnetic field generated by the horizontal direction magnetic field correction coil 2, an external magnetic field (in a tube axial direction) with respect to the CRT (not shown) is forcibly corrected to be equal to the external magnetic field for the place at which the CRT has been adjusted (place of manufacturing).
The above constitution of the conventional external magnetic field correction device of a CRT display device presents the following drawbacks.
In the example of FIG. 1, the operation of the offset voltage generating circuit 7 needs to perform adjustment for each individual product so that it takes a long time for the adjustment and thus resulting in higher manufacturing costs.
In case the place of arrangement or the orientation of the CRT display device has been changed, it is required to degauss the whole display device including the CRT by using an internal degaussing circuit or an external device in order to make the CRT display device present maximum performance.
When an internal degaussing circuit or an external degaussing device is operated, an abnormally large magnetic field is detected for a peripheral magnetic field of the horizontal direction magnetic field sensing element 4 as compared to a normal external magnetic field such as geomagnetism so that malfunctions may occur in the case of FIG. 1.
In other words, the smoothing circuit 9 as described in the example of FIG. 1 is effective to disturbance levels such as high harmonic noise that is impressed at normal operation and is required to be operated within an operational response time (within a few seconds) that is without hindrance when placing environment is changed or power source is inputted;
however, it can not eliminate an influence of an abnormally large magnetic field which may be generated when an internal degaussing circuit or an external degaussing device is operated.
Therefore, when malfunctions of the magnetic field correction device occurs while an internal degaussing circuit or an external degaussing device is operated so that abnormal current runs through the correction coil, it may cause a drawback in that the display device is reversely magnetized so that it may remarkably degrade the performance of the external magnetic field correction device.
On the other hand, the problem of malfunctions due to operations of an internal degaussing circuit or an external degaussing device that arises in the conventional art of FIG. 1 also exists for the example of FIG. 2.
In the manufacturing stage (process of combining/assembling with the deflection coil) of the CRT (not shown), when adjusting an arranging position of the deflection coil (not shown) or electric magnetic characteristics, it is required to set irregularities in offset current originating in operational irregularities of the magnetic field detector, difference detector or amplifier to be less than a regular value.
This is to assure assembling/adjusting conditions (peripheral magnetic field) for designing a CRT or deflection yoke.
In other words, although it seems that the conventional art according to FIG. 2 solves, by the provision of a memory and a difference detector, each of the irregularities in outputs of the magnetic field detector which presents a problem after the product has been shipped, it does still not solve the problem of initial deviation which presents a problem at the time of assembling the product.
Therefore, it is required to adjust offset current also.